1. Field of the Invention
This invention relates to a method for modifying the shape selectivity of natural ferrierite in the catalytic conversion of hydrocarbons so that hydrocarbons, other than normals, are converted, e.g. cracked selectively.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction within which there are a large number of smaller cavities which may be interconnectd by a number of still smaller channels or pores. These cavities and pores are precisely uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation for example, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li is equal to unity. One type of cation may be exchanged either entirely or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formulation of a great variety of synthetic aluminosilicates. These aluminosilicates have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-35 (U.S. Pat. No. 4,016,245) and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
It is well known in the prior art that certain zeolites such as zeolites A, X. or Y and the naturally occurring zeolites chabazite, erionite, faujasite and mordenite can be stability-enhanced by methods comprising ion exchange and/or heating. U.S. Pat. Nos. 3,354,077; 3,375,065; and 3,402,966 describe such methods. U.S. Pat. No. 3,997,474, on the other hand, describes a method for activating naturally occurring ferrierite to make it selective primarily for cracking normal paraffins by treatment with ammonium ions and HF.